Solution processing of kesterite semiconductors

ABSTRACT

Methods for depositing a kesterite film comprising a compound of the formula: 
       Cu 2−x Zn 1+y Sn(S 1−z Se z ) 4+q , 
     wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1, generally include contacting a hydrazine-based solvent, a source of Cu, a source of Sn, a source of Zn carboxylate, a source of at least one of S and Se, under conditions sufficient to form a solution substantially free of solid particles; applying the solution onto a substrate to form a thin layer; and annealing the thin layer at a temperature, pressure, and length of time sufficient to form the kesterite film. Also disclosed are hydrazine-based precursor solutions for forming a kesterite film and a photovoltaic device including the kesterite film formed by the above method.

DOMESTIC PRIORITY

The present application is a DIVISIONAL of U.S. application Ser. No.13/644,672, filed on Oct. 4, 2012, the contents of which areincorporated herein by reference in its entirety.

BACKGROUND

The present disclosure relates to a method of depositing a kesteritefilm. More particularly, the present disclosure relates to a method ofdepositing a kesterite film from a precursor solution.

Large-scale production of photovoltaic devices requires high-throughputtechnologies and abundant environmentally friendly materials. Thin-filmchalcogenide-based solar cells provide a promising pathway to costparity between photovoltaic and conventional energy sources.

Currently, only Cu(In,Ga)(S,Se)₂ and CdTe technologies have reachedcommercial production and offer over 10 percent power conversionefficiency. These technologies generally employ (i) indium andtellurium, which are relatively rare elements in the earth's crust, or(ii) cadmium, which is a highly toxic heavy metal.

Copper-zinc-tin-chalcogenide kesterites have been investigated aspotential alternatives because they are based on readily available andlower cost elements. However, photovoltaic cells with kesterites, evenwhen produced using high cost vacuum-based methods, at best only <6.7percent efficiencies, see Katagiri, H. et al. Development of CZTS-basedthin film solar cells; Thin Solid Films 517, 2455-2460 (2009).

The commonly owned applications: U.S. Pub. App. No. 2011/0094557A1, andPCT App. No. WO 2011/051012 to Todorov et al. and a publication by T.Todorov, K. Reuter, D. B. Mitzi, Advanced Materials, (2010) Vol. 22,pages 1-4, generally describe a hydrazine-based deposition approach ofdepositing homogeneous chalcogenide layers from mixed slurriescontaining both dissolved and solid metal chalcogenide speciesdispersions of metal chalcogenides in systems that do not requireorganic binders. Upon anneal the particle-based precursors readily reactwith the solution component and form large-grained films with goodelectrical characteristics. Recently, this process achieved world-recordefficiency for this class of materials of 11.1% (T. Todorov, J. Tang, S.Bag, O. Gunawan, T. Gokmen, Y. Zhu, D. B. Mitzi, “Beyond 11% Efficiency:Characteristics of State-of-the-Art Cu2ZnSn(S,Se)4 Solar Cells”,Advanced Energy Materials, early view: DOI: 10.1002/aenm.201200348).

A major challenge in hydrazine-based copper-zinc-tin-chalcogenidekesterite processing including copper-zinc-tin-sulfide (CZTS),copper-zinc-tin-selenide (CZTSe), and copper-tin-zinc-sulfur-selenium(CZTSSe), is the poor solubility of the zinc chalcogenide-hydrazinatesthat generally form a solid phase in the ink. Unlike the various solublechalcogenides compounds, zinc compounds such as ZnS and ZnSe, togetherwith most transition metals and metal chalcogenides, show negligiblesolubility in hydrazine-based solvent systems. The morphology anddispersibility of the solid phase of these zinc compounds are difficultto control resulting in poor reproducibility of the hydrazine-basedcopper-zinc-tin-chalcogenide kesterite slurries that may causemicro-scale compositional non-uniformities, thereby potentiallydeteriorating device performance. Furthermore, particle-based inks mayhave poor compatibility with liquid-coating equipment such asslit-casting and spin coating due to non-Newtonian liquid properties ofthese slurries.

A pure solution precursor ink formulation forcopper-zinc-tin-chalcogenide kesterite based on DMSO solutions waspreviously reported (W. Ki, H. Hillhouse Adv. Energy Mater. 2011, 1,732-735). However, maximum efficiency reached only 4.1% possibly due todifficult to eliminate impurities introduced with the selectedprecursors. Another example employing sol-gel solutions inmethoxyethanol reports 2.2% efficiency.

SUMMARY OF THE INVENTION

Accordingly, the present disclosure discloses various methods fordepositing a kesterite film; a hydrazine-based precursor solution forforming the kesterite film; and photovoltaic devices including thesolution deposited kesterite film.

In one embodiment, a method of depositing a kesterite film comprising acompound of the formula:

CU_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q),

wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1, said method comprising contacting ahydrazine-based solvent, a source of Cu, a source of Sn, a source of Zncarboxylate, a source of at least one of S and Se, under conditionssufficient to form a solution substantially free of solid particles;applying the solution onto a substrate to form a thin layer; andannealing the thin layer at a temperature, pressure, and length of timesufficient to form the kesterite film.

In another embodiment, a method of depositing a kesterite filmcomprising a compound of the formula:

CU_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q)

wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1,said method comprising contactinghydrazine, a source of Cu, and a source of at least one of S and Seforming solution A; contacting hydrazine, a source of Sn, a source of atleast one of S and Se, and a source of Zn forming dispersion B; mixingsaid solution A and said dispersion B under conditions sufficient toform a solution substantially free of particles; applying said solutiononto a substrate to form a thin layer; and annealing the thin layer at atemperature, pressure, and length of time sufficient to form saidkesterite film.

A hydrazine-based precursor solution for forming a kesterite film,comprises a source of Cu, a source of Sn, a source of Zn carboxylate, asource of at least one of S and Se; and hydrazine, wherein a dispersionof the Zn carboxylate in hydrazine is mixed with a solution comprisinghydrazine and the source of Sn to solubilize and stabilize the source ofthe Zn carboxylate.

A photovoltaic device, comprising a top electrode having transparentconductive material; an n-type semiconducting layer; a kesterite film onsaid substrate formed by the method in claim 1; and a substrate havingan electrically conductive surface.

The disadvantages associated with the prior art are overcome by thepreferred embodiments of the present invention in which pure CZTSprecursor solution substantially free of solid particles is employed.

The disclosure may be understood more readily by reference to thefollowing detailed description of the various features of the disclosureand the examples included therein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring now to the figures wherein the like elements are numberedalike:

FIG. 1 is an X-ray diffraction pattern of mixed S—Se and pure sulfidekesterite materials prepared in Examples 1 and 2.

FIG. 2 is a cross-sectional scanning electron microscopy image of a filmprepared according to Example 1.

FIG. 3 is a cross-sectional scanning electron microscopy image of a filmprepared according to Example 2.

FIG. 4 is a cross-sectional scanning electron microscopy image of a filmprepared according to Example 3.

DETAILED DESCRIPTION

The present disclosure relates to a method of depositing ahydrazine-based copper-zinc-tin-chalcogenide kesterite film having Cu,Zn, Sn, and at least one of S and Se, and more particularly to asolution deposition method of kesterite-type Cu—Zn—Sn—(Se,S) materialsto form a film and improved photovoltaic devices based on these films.

The method generally includes forming a solution including ahydrazine-based solvent, a source of Cu, a source of Sn, a source of atleast one of S and Se, and a source of Zn carboxylate, wherein thesolution is substantially free of solid particles; applying the solutiononto a substrate to form a thin layer; and annealing at a temperature,pressure, and length of time sufficient to form the kesterite film.

The hydrazine-based solvent includes hydrazine in an amount from about50% by weight to about 100% by weight, amounts of about weight 70% byweight to about 100% by weight in other embodiments, and about 90% byweight to about 100% by weight in still other embodiments. In additionto the hydrazine, the solvent can further include an organic orinorganic solvent. The ink solution may also include at least oneadditive each containing a metal selected from: Li, Na, K, Mg, Ca, Sr,Ba, Sb, Bi, and a combination thereof, wherein the metal is present inan amount from about 0.01 weight % to about 5 weight %.

The kesterite film formed by the process can be represented by formula(I):

CU_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q),  (I)

wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1.

In one embodiment, the kesterite has the above formula wherein x, y, zand q respectively are: 0≦x≦0.5; 0≦y≦0.5; 0≦z≦1; −0.5≦q≦0.5.

In one embodiment, the source of Cu is at least one of Cu, Cu₂S andCu₂Se; the source of Sn is at least one of Sn, SnSe, SnS, SnSe₂, SnS₂,Sn formate and Sn acetate; the source of Zn carboxylate is at least oneof zinc acetate and zinc formate; the source of S is selected from:elemental sulfur, CuS, Cu₂S, SnS, SnS₂, ZnS, and a mixture thereof; andthe source of Se is selected from at least one of elemental Se, SnSe₂,and SnSe.

In one embodiment, the method of depositing the hydrazine-basedcopper-zinc-tin-chalcogenide kesterite film includes contactinghydrazine, a source of Cu, and a source of at least one of S and Seforming solution A; contacting hydrazine, a source of Sn, a source of atleast one of S and Se, and a source of Zn forming dispersion B; mixingthe solution A and dispersion B under conditions sufficient to form asolution substantially free of solid particles; applying the resultingsolution onto a substrate to form a thin layer; and annealing at atemperature, pressure, and length of time sufficient to form thekesterite film. While not wanting to be bound by theory, it is believedthat the presence of the tin chalcogenide ions promotes stabilization ofthe zinc in the solution.

The step of applying in the method of the present disclosure ispreferably carried out by a method selected from: spin coating, dipcoating, doctor blading, curtain coating, slide coating, spraying, slitcasting, meniscus coating, screen printing, ink jet printing, padprinting, flexographic printing, and gravure printing.

The substrate is selected from: metal foil, glass, ceramics, aluminumfoil coated with a layer of molybdenum, a polymer, and a combinationthereof. In one embodiment, the substrate is coated with a transparentconductive coating.

The step of annealing is preferably carried out at a temperature fromabout 200° C. to about 800° C. and ranges there between. In otherembodiments, the annealing temperature is from about 400° C. to about600° C. and in still other embodiments, the anneal temperature is fromabout 500 to about 600° C. The step of annealing is typically carriedout in an atmosphere including: at least one of N₂, Ar, He, forming gas,and a mixture thereof. This atmosphere can further include vapors of atleast one of: sulfur, selenium, and a compound thereof. In mostembodiments, the annealing step is carried out at an appropriatetemperature that is high enough for thermal decomposition of theprecursor but low enough to maintain the resulting film in an amorphousstate. Typically, the annealing step is for an amount of time of about 1second to about 60 minutes. More typically, the annealing step is forabout 30 sec to about 20 minutes. The step of annealing can be carriedour by any technique known to the skilled in the art, including but notlimited to: furnace, hot plate, infrared or visible radiation, e.g.,laser, lamp furnace, rapid thermal anneal unit, resistive heating of thesubstrate, heated gas stream, flame burner, electric arc and plasma jet.

The hydrazine-based precursor solution in accordance with the presentdisclosure provides greater versatility to the end user to tailor theparticular stoichiometry of the kesterite film by adjusting the ratiosof Cu/Sn/Zn/(S, Se) sources without the need for long range diffusion aswould be expected for slurry based systems. As a result, high qualityand highly pure kesterite films can be obtained.

The thickness of the applied hydrazine-based kesterite precursor layergenerally ranges from about 0.2 microns to about 5 microns in mostembodiments. In other embodiments, the thickness generally ranges fromabout 0.5 microns to about 3 microns, and in still other embodiments,the thickness ranges from about 1 microns to about 2.5 microns.

Thus, the method of the present disclosure produces a composition whichincludes a solution containing hydrazine solvent, a source of Cu, asource of Sn, a source of Zn carboxylate, and a source of at least oneof S and Se, which when annealed, forms a compound of the formula:Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q) wherein 0≦x≦1; 0≦y≦1; 0≦z≦1;−1≦q≦1; and preferably a compound of the above formula wherein x, y, zand q respectively are: 0≦x≦0.5; 0≦y≦0.5; 0≦z≦1; −0.5≦q≦0.5.

The present disclosure further provides a photovoltaic device,including: a substrate having an electrically conductive surface; akesterite film on the substrate formed by the method of the presentdisclosure; an n-type semiconducting layer; and a top electrode having atransparent conductive material. The substrate can be glass, plastic,polymer, ceramic, or aluminum foil, and can be coated with a molybdenumlayer; the n-type semiconducting layer has at least one of: ZnS, CdS,InS, oxides thereof, and selenides thereof; and the transparentconductive material can be doped ZnO, indium-tin oxide (ITO), doped tinoxide, or carbon nanotubes.

For example, photovoltaic cells may be constructed, incorporating thesolution deposition methods of this disclosure, by layering the metalchalcogenide with other materials to form a two terminal,sandwich-structure device. For example, one could form a layer ofCu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q) wherein 0≦x≦1; 0≦y≦1; 0≦z≦1;−1≦q≦1 deposited as disclosed herein on top of a metal contact, such asmolybdenum (Mo), which is supported on a rigid or flexible substrate(e.g., glass, metal, plastic). TheCu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q) layer could then be covered witha buffer layer, which can be a metal chalcogenide such as CdS or ZnSe oran oxide such as TiO₂. This buffer layer could be deposited in the samefashion as the Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q) layer using anyof the methods of the present disclosure or it could be deposited moreconventionally (e.g. by chemical bath or vapor deposition techniques).The buffer layer would then be covered with a transparent top contactsuch as doped TiO₂, indium tin oxide, or fluorine-doped tin oxide,completing the photovoltaic cell.

Alternatively, the photovoltaic cell could be constructed in the reverseorder, using a transparent substrate (e.g. glass or plastic) supportinga transparent conducting contact (such as doped TiO₂, indium tin oxide,or fluorine-doped tin oxide). The buffer layer would then be depositedon this substrate and covered with the metal chalcogenide layer (such asCu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q)), and finally with a backcontact (such as Mo or Au). In either case, the metal chalcogenide(“absorber”) layer could be deposited by the solution deposition methodsdescribed in this disclosure.

The present disclosure further provides a photovoltaic module thatincludes a plurality of electrically interconnected photovoltaic devicesdescribed in the present disclosure.

Experimental studies of zinc carboxylate dissolution in hydrazineindicated that slurries obtained by mixing of zinc acetate or zincformate in hydrazine can be successfully dissolved by addition oftin-chalcogenide hydrazine solutions. The rest of the necessaryprecursors, such as Cu and other chalcogens can be added either to theinitial tin solution or at a later stage. In contrast, the followingalternative dissolution routes for zinc carboxylate lead in all cases toinsoluble product: (i) mixing with pure hydrazine, (ii) mixing withchalcogen-hydrazine solutions, (iii) mixing with chalcogen-coppersolutions. It is believed that the action of the tin chalcogenidehydrazinate ions provides zinc stabilization in solution. Ratios of(S,Se)/Sn from 3 to 4 were found suitable for solution formation whilehigher ratios promoted precipitate formations.

Pure sulfide solutions were found to be more stable than selenidesolutions with identical molar composition.

Advantageously, the present disclosure provides a new approach tosolubilize zinc species in hydrazine-based kesterite precursor inks. Thehydrazine-based inks are substantially free of particles and providegreater versatility to tailor the stoichiometry of the kesterite filmwithout secondary phases being present. The inks can be used in a broadrange of semiconductor devices, although they are especially effectivein light receiving elements such as photodiodes and photovoltaic cells.

The following examples are presented for illustrative purposes only, andare not intended to limit the scope of the invention.

Example 1 Mixed S—Se Kesterite Solution, Film and Device Thereof

Zinc formate, 0.36 grams (g) was dispersed in 1 milliliter (ml)hydrazine (Slurry A). Tin powder, 0.25 g and Se, 0.75 g were dissolvedin 3 ml hydrazine (Solution B). Copper powder, 0.226 g and sulfur, 0.175g were dissolved in 1.5 ml hydrazine (Solution C). Solution B was addedto Slurry A, followed by Solution C and 1 ml hydrazine, formingdeposition ink D.

Six consecutive layers were spin coated at 600 revolutions per minute(rpm) on a molybdenum-coated glass and annealed on a covered hot plateat a maximum temperature above 540° C.

Solar cells were fabricated from the above-described Cu₂ZnSn(Se,S)₄films by deposition of 60 nanometers (nm) CdS buffer layer by chemicalbath deposition, 100 nm insulating ZnO and 130 nm ITO (indium-doped zincoxide) by sputtering, followed by Ni/Al metal contacts deposited byelectron-beam evaporation.

Device photovoltaic efficiency measured at 1.5 AM conditions was 6.8%,with Voc=0.404 V, Jsc=28.9 mA/cm², Fill Factor=58.2%.

FIG. 1 provides X-ray diffraction patterns of the obtained S—Sekesterite film (top). FIG. 2 provides a scanning electron micrograph ofthe S—Se kesterite film. Large-grain void-free layers was observed,which is generally considered desirable for photovoltaic devices.

Example 2 Pure S-Kesterite Solution, and Film Thereof

Zinc formate, 0.37 g was dispersed in 1 ml hydrazine (Slurry E). Tinpowder, 0.26 g and S, 0.312 g were dissolved in 3 ml hydrazine (SolutionF). Copper powder, 0.226 g and sulfur, 0.175 g were dissolved in 1.5 mlhydrazine (Solution G). Solution F was added to Solution E, followed bySolution G and 1 ml hydrazine, forming deposition ink H.

Six consecutive layers were spin coated at 600 rpm on amolybdenum-coated glass and annealed on a covered hot plate at a maximumtemperature above 540° C.

FIG. 1 provides X-ray diffraction patterns of the obtained pure sulfidekesterite film (bottom). FIG. 3 provides a scanning electron micrographof the pure sulfide kesterite film. Large-grain void-free layers wasobserved, which is generally considered desirable for photovoltaicdevices.

Example 3 Improved S—Se Kesterite Solutions, Film and Device Thereof

Zinc formate, 0.735 g was dispersed in 1.5 ml hydrazine (Slurry I). Tinpowder, 0.52 g and Se, 1.21 g were dissolved in 5 ml hydrazine (SolutionJ). Copper powder, 0.436 g and sulfur, 0.33 g were dissolved in 3 mlhydrazine (Solution K). Solution J was added to Solution K followed by 1ml of hydrazine used to wash vial J resulting in solution L. Solution Lwas added to slurry I followed by 1 ml of hydrazine used to wash vial L,forming deposition ink M. One (1) ml of ink M was added to 0.1 ml HZcontaining 0.07 g Se forming deposition ink N.

On a molybdenum-coated glass, one layer of ink M was spun at 800 rpmfollowed by six layers spun at 600 rpm and one layer of ink N spun at500 rpm and annealed on a covered hot plate at a maximum temperatureabove 540 C.

Solar cells were fabricated from the above-described Cu₂ZnSn(Se,S)₄films by deposition of 60 nm CdS buffer layer by chemical bathdeposition, 100 nm insulating ZnO and 130 nm ITO (indium-doped zincoxide) by sputtering, followed by Ni/Al metal contacts deposited byelectron-beam evaporation.

Device photovoltaic efficiency measured at 1.5 AM conditions was 10.4%,with Voc=0.478 V, Jsc=33.8 mA/cm², Fill Factor=64.4%.

FIG. 4 provides a scanning electron micrograph of the S—Se kesteritefilm. Large-grain void-free layers was observed, which is generallyconsidered desirable for photovoltaic devices.

Film Characterization

X-ray diffraction patterns of the obtained films matched kesterite phase(FIG. 1). SEM images indicate large-grain void-free layers desirable forphotovoltaic devices (FIGS. 2, 3).

The present invention has been described with particular reference tothe preferred embodiments. It should be understood that variations andmodifications thereof can be devised by those skilled in the art withoutdeparting from the spirit and scope of the present invention.Accordingly, the present invention embraces all such alternatives,modifications and variations that fall within the scope of the appendedclaims.

What is claimed is:
 1. A photovoltaic device, comprising: a topelectrode having transparent conductive material; an n-typesemiconducting layer; a kesterite film comprising a compound of theformula: Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q), wherein 0≦x≦1; 0≦y≦1;0≦z≦1; −1≦q≦1, and formed by contacting a hydrazine-based solvent, asource of Cu, a source of Sn, a source of Zn carboxylate, a source of atleast one of S and Se, under conditions sufficient to form a solutionfree of solid particles, wherein the conditions consist of dispersingthe source of the zinc carboxylate in a portion of the hydrazine-basedsolvent to form a first slurry of zinc carboxylate particles, anddispersing the source of Sn in a portion of the hydrazine-based solventto form an initial Sn solution, and mixing the first slurry and theinitial Sn solution to dissolve the zinc carboxylate particles and formthe solution free of solid particles, wherein the source of Cu, and thesource of at least one of S and Se are added to the initial Sn solutionor to the solution free of solid particles; applying the solution freeof solid particles onto a substrate to form a thin layer having athickness of about 0.2 microns to about 5 microns; and annealing thethin layer at a temperature, pressure, and length of time sufficient toform the kesterite film; and the substrate having an electricallyconductive surface.
 2. The photovoltaic device of claim 1, wherein: saidsubstrate is coated with a molybdenum layer and is selected from thegroup consisting of glass, plastic, polymer, ceramic, and aluminum foil;said n-type semiconducting layer has at least one of: ZnS, CdS, InS,oxides thereof, and selenides thereof; said transparent conductivematerial is selected from the group consisting of: doped ZnO, Indium-tinoxide, doped tin oxide, and carbon nanotubes.
 3. The photovoltaic deviceof claim 1, comprising: a plurality of electrically interconnectedphotovoltaic devices forming a photovoltaic module.